Timing synchronization between nodes in a network is described in various standards such as IEEE 1588-2008 “Standard for a Precision Clock Synchronization Protocol for Networked Measurement and Control Systems,” ITU-T G.8265.1/Y.1365.1 “Precision time protocol telecom profile for frequency synchronization,” ITU-T G.8275.1/Y1369.1 “Precision time protocol telecom profile for phase/time synchronization with full timing support from the network,” the contents of each is incorporated by reference herein. The requisite information for the transfer of precise time is (1) a time reference point, or “significant instant” to which timing information can be related, (2) the timing information itself, and (3) a measure of the delay it takes to transfer the timing information between two nodes. IEEE 1588-2008 is referred to as Precision Time Protocol (PTP) and is used to synchronize clocks throughout the network. IEEE 1588 facilitates time synchronization by transferring time information in packets between network nodes. To synchronize time, a master clock sends time information to a slave clock. In addition, a round trip delay measurement is used to estimate the delay between the master clock and the slave clock. With the time information from the master and an estimate of the packet delay, the slave clock can synchronize its local time to the master clock. Because a round trip delay measurement is used to estimate the one-way delay, the achievable accuracy of time synchronization at the slave clock is dependent upon the forward and reverse path delays being equal. Any difference between the forward and reverse path delays, known as delay asymmetry, will result in a time error if it is not compensated for.
Systems can compensate for delay asymmetry introduced by optical modules if the delay is static. This is typically acceptable for “gray” client optical modules (i.e., QSFP28 LR4) because these modules do not introduce significant amounts of delay. However, some advanced optical modules may introduce dynamic delays. For example, Digital Coherent Optical (DCO) or Coherent Consortium for On-Board Optics (COBO) modules may map the client signal to an asynchronous server layer such as Optical Transport Network (OTN) or in the case of the Optical Internetworking Forum (OIF) ZR, there is an asynchronous remapping of Alignment Markers (AM). The OIF ZR optical modules typically contain complex Physical Medium Attachment (PMA)/Physical Medium Dependent (PMD) functions and are modeled like an Ethernet extension sub-layer. Additionally, coherent optical modules are likely to employ Soft Decision Forward Error Correction (SD-FEC). These processes can introduce delay asymmetry and uncertainty that is dynamic and unpredictable, leading to inaccurate timing.